Patent GB-1043457-A (GB)
Title: Correcting circuits for colour television signals
Inventor: WOOD CHARLES BARRINGTON BISHOP
IPC: H04N9/11
Filed: 1964-08-10
Abstract: 1,043,457. Colour television. BRITISH BROADCASTING CORPORATION. July 2, 1965 [Aug. 10, 1964], No. 32529/64. Heading H4F. The invention relates to correcting circuits for colour television signals and particularly to such signals derived from colour film. As shown, Fig. 6, colour separation signals derived by scanning a colour film, are applied to input terminals 10, 11 and 12 respectively which are connected by separate channels to output terminals 13, 14 and 15 respectively, blanking pulses synchronized with the blanking intervals being applied at an input 16. The " colour gain " section which is used to vary the gain of the three channels to compensate for colour casts visible primarily in the low density parts of the film, i.e. the picture highlights, comprises two controls. Considering the red channel, one of the controls is a twelve-position switch RSW1, ganged to corresponding switches in the other channels, which selects a colour channel, or a suitable combination of two colour channels, and hence an effective colour axis along which increased gain may be applied, the other control being a separate gain control RR1, ganged to corresponding controls in the other channels, which determines the amount of correction, i.e. the magnitude of the gain change. The twelve positions of switch RSW1 represent vectors 30 degrees apart on the colour triangle, red, green and blue being 120 degrees apart, and Fig. 4 illustrates the colour axis associated with each switch position and the coefficients determining the contributions of the primaries to each of the twelve colours. The variations of. gain in the colour-separation signal channels according to the coefficients shown in Fig. 4 are determined by resistors RR3, RR4, RR5 (in the red channel) associated with the twelveposition switch RSW1 and the magnitude of the correction is controlled by the three-gang potentiometer RR1. When the slider of RR1 is connected to the input signal terminal 10, maximum correction is applied, RB being an amplifier having a low input impedance so that a variation in the effective value of RR2 causes a change in the signal current that flows into amplifier RB when correction is applied. Resistors RR3, RR4 and RR5 have values such that the necessary gain changes in the red channel are effected at each switch position according to the coefficients in Fig. 4. When the slider of RR1 is connected to earth, no connection is applied and there is no overall gain change since the input impedance of the amplifier RB is low compared with RR5. Thus the twelve-position, three-gang switch RSW1 &c. and-the three-gang potentiometer RR1 &c. allow the white of the film to be corrected, i.e. compensate for " white crushing." To compensate for "black-stretch" or "black-crush" " the signals derived from the film are passed to variable gain devices which operate non-linearly, referred to as " colour gamma " in Fig. 6. The amplifier RB feeds two non-linear circuits 18 and 19, having black-crush and black-stretch characteristics respectively, Fig. 5 (not shown). The two non-linear signals are available at either end of a potentiometer RR6 and movement of the slider of this potentiometer permits a smooth transition from maximum black-stretch to maximum black-crush through a central position where the effective transfer characteristic of the complete circuit is approximately linear. As for the colour gain control, the twelveposition three-gang switch RSW2 and resistors RR7, RR8 and RR9 select the colour axis along which non-linear compensation is applied. Defects caused by excessive or inadequate general density of the film are compensated for by adjustment of a master lift control 17 and a master gain control RA, GA, BA. The blanking pulses, shown as negative going, are applied from 16 through a circuit D which provides positive and negative going pulses at its two outputs and these pulses are applied to the ends of a potentiometer 17. Circuits D and 17 constitute the master lift control by means of which the D.C. component of all colour separation signals may be varied equally. The slider of 17 adjusting the magnitude and sense of the pulses which are combined with the signals during the blanking intervals in combining circuits RC, GC and BC. The master gain control RA, GA, BA comprises three controls ganged together each comprising a variable attenuator followed by an amplifier of fixed gain.